STACK Science: Developing Slow- and Fast-Twitch Muscle Fibers

In our first STACK Science post, How Muscles Work and How to Make Them Work Better, we told you that muscles are composed of individual muscle cellsâcalled muscle fibersâthat are able to shorten to effect a contraction in the muscle. Although the goal of every muscle fiber is to shorten when stimulated by the nervous system, the fibers are specialized to produce either short bursts of strength, for sports like weightlifting or sprinting, or longer term energy for endurance activity, like walking or jogging.

Fiber Types
There are two kinds of fibers, Type I and II, more commonly referred to as slow- and fast-twitch, respectively. Type II fibers produce fast and strong contractions, making them the primary contributors to strength and power. They are characterized by their ability to use limited fuel stores via the anaerobic energy system for short bursts of energyâup to three minutes in length. After three minutes, Type I fibers take over, producing less powerful, but more sustained, muscle contractions. They use oxygen along with carbohydrate, fat and protein in the aerobic energy system to create a constant supply of energy for endurance sports.

In our first STACK Science post, How Muscles Work and How to Make Them Work Better, we told you that muscles are composed of individual muscle cells—called muscle fibers—that are able to shorten to effect a contraction in the muscle. Although the goal of every muscle fiber is to shorten when stimulated by the nervous system, the fibers are specialized to produce either short bursts of strength, for sports like weightlifting or sprinting, or longer term energy for endurance activity, like walking or jogging.

Fiber Types
There are two kinds of fibers, Type I and II, more commonly referred to as slow- and fast-twitch, respectively. Type II fibers produce fast and strong contractions, making them the primary contributors to strength and power. They are characterized by their ability to use limited fuel stores via the anaerobic energy system for short bursts of energy—up to three minutes in length. After three minutes, Type I fibers take over, producing less powerful, but more sustained, muscle contractions. They use oxygen along with carbohydrate, fat and protein in the aerobic energy system to create a constant supply of energy for endurance sports.

How to Train
Athletes should train their muscles the way they are used in their sports. Athletes in power sports like football, hockey and basketball should focus on developing Type II muscle fibers through explosive exercises, such as Olympic lifts, Squats and plyometrics. This type of training improves strength, size and power critical for running, jumping and other powerful movements. Power athletes should avoid long and slow endurance training, which can actually reduce muscle size and strength.

Endurance athletes should primarily develop their Type I muscle fibers with time-extended or high-intensity aerobic exercise, or circuit training that includes exercises involving the body's larger muscle groups, like Push-Ups and Squats. Also, Fartlek training—a combination of high and low intensity aerobic exercise—and interval training, with circuits of three to five minutes of near maximum capacity exercise followed by three to five minutes of rest, are proven methods for improving aerobic conditioning.